The field of the invention is positron emission tomography (PET) scanners, and particularly the calibration of such scanners.
Positrons are antimatter electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides commonly employed in diagnostic imaging are fluorine-18 (.sup.18 F), carbon-11 (.sup.11 C), nitrogen-13 (.sup.13 N), and oxygen-15 (.sup.15 O). These are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected in the patient and become involved in such processes as glucose metabolism, fatty acid metabolism and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, a matter-antimatter annihilation converts them into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners--each gamma ray has an energy of 511 keV and the two gamma rays are directed in substantially opposite directions. An image is created by determining the number of such annihilation events at each location within the field of view.
The PET scanner includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes millions of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers are employed to reconstruct an image using well known computed tomography techniques.
One of the vital calibration operations in a PET scanner is the coincidence timing calibration. The purpose of this calibration is to correct for relative timing differences in the detection modules and the "front end" electronics of the PET scanner. This calibration is traditionally performed by placing a source of positrons at the center of the detector rings and adjusting each channel of the front end electronics manually until the coincident events registered by each channel is maximized and relatively uniform around the entire ring. This is a time consuming iterative process in which many manual adjustments are made and long data collection times are required.
More recently, a PET scanner has been proposed in which the time difference between the signals which record coincidence events can be maintained by the scanner. Since the photons travel an equal distance to the two detectors which record a coincidence event, their signals should indicate an event at precisely the same moment. Any difference in time, therefore, represents an error caused by differences in the detectors, PMTs or electronic circuits. As described in co-pending U.S. application Ser. No. 920,350 entitled "Sorter For Coincidence Timing Calibration In A PET Scanner," the time difference information may be gathered and stored as a sinogram calibration array when the PET scanner is operated in a calibration mode. These time differences can be offset by introducing appropriate delays in the scanner's front end electronics.